The present invention relates to a method of and an apparatus for continuously monitoring the concentration of a specified radioactive element in a gas, and more particularly to a method of and an apparatus for monitoring the radioactivity concentration of a gas wherein a radioactive element in the gas is accumulated on an absorption material and then the radioactivity of the accumulated element is measured, so that even an extremely low radioactivity concentration can be detected.
An article entitled "Continuous Monitoring of Radioactive Iodine Emission" by J. G. Wilhelm and H. Mahnau, which was presented at the 13th AEC Air Cleaning Conference, 1974, discloses a monitoring apparatus in which a sample gas is introduced into a filter cartridge filled with an iodine absorption material, to accumulate .sup.131 I in the sample gas on the absorption material, .gamma.-rays emitted from the accumulated .sup.131 I are detected by a scintillation counter disposed adjacently to the filter cartridge, and both the frequency of detecting .gamma.-rays and the volume of the sample gas passing through the filter cartridge are monitored to know the radioactivity concentration of .sup.131 I in the sample gas.
In such an apparatus, it is an important problem how many portions of .sup.131 I isotopes can be collected by the absorption material when the sample gas passes through the filter cartridge. The absorbing power of the iodine absorption material depends upon such factors as temperature, humidity and velocity of the gas passing through the filter cartridge. Accordingly, the ratio of the amount of iodine collected by the filter cartridge to the amount of iodine in the gas passing through the filter cartridge varies with the above-mentioned factors. Therefore, the radioactivity concentration of a gas which is calculated on the basis of the radioactivity of accumulated iodine, contains unavoidable errors.